Known energy resources such as deposits of fossil fuel are occasionally left untapped or underutilized for a variety of reasons. A deposit of fossil fuel may, for example, be located at a site that is remote from an appropriate transportation system, such as a railway or pipeline, for transporting the fuel to an energy utilization center, such as a power plant. Even a readily-accessible deposit of fossil fuel may remain untapped or underutilized if the deposit is of such small capacity or of low energy content as to negate profitable exploitation.
Fossil fuel markets are prone to occasional fluctuations that mitigate against sustained exploitation of marginally profitable deposits. Typically, such marginal deposits are alternately exploited and left idle in response to market cycles of shortage and oversupply. It would be desirable to provide an alternative use for marginal deposits of fossil fuels during periods of oversupply in order to maintain sustained production and hence to obviate the expense of alternately mothballing and re-tapping such deposits.
Low energy content natural gas is a good example of an occasionally underutilized energy resource. It is a standard requirement in the natural gas industry that pipeline-quality natural gas possess an energy content on the order of 1,000 Btu/ft.sup.3. A consequence of this energy requirement has been underutilization of gas fields which produce natural gas having an energy content of less than about 800 Btu/ft.sup.3. Where it has been economically feasible, such low-quality gas has been blended with gas having an energy content in excess of 1,000 Btu/ft.sup.3 in order to obtain a product meeting the standard. Such blended gas, however, may contain significant quantities of undesirable impurities and is not preferred by industrial gas consumers. It is also often the case that low-quality gas is available in gas fields that are geographically remote or of such low BTU capacity that blending such gas with higher-quality gas would not justify the expense of obtaining rights-of-way and constructing pipelines to carry the gas from the fields to a suitable site for blending.
While fossil fuels are occasionally underutilized, electric utilities have generally experienced a steady decline in the excess generating capacity needed in order to meet peak electrical demand. It has also become increasingly difficult for electric utilities to overcome social and economic barriers to the construction of large, base generating units. It would be desirable to provide a system for generating electrical energy which could be conveniently deployed during periods of peak demand. A system that could provide incremental units of supplementary base generating capacity as needed would also be desirable.
Combustion turbine/generator units are widely used for peak power generation. Combustion turbines are known which use such fuel sources as natural gas, petroleum, or finely divided, particulate material. Gas-fueled combustion turbine/generator units have become a particularly attractive means for generating electrical energy since they may be more rapidly brought online than other types of generating units. Natural gas also tends to be less polluting and more economical than other combustion turbine fuels.
Gas-fueled combustion turbines operate with optimal efficiency at rotational speeds which range between 4,000 RPM and 20,000 RPM depending upon the type of turbine and the load which the turbine is required to drive. In order to provide AC power at a standard frequency of 60 Hz with standard generating equipment, it is necessary to regulate the rotational speed of the generator to precisely 3,600 RPM. Hence systems employing gas-fueled combustion turbines to drive conventional generators may suffer from inherently sub-optimal energy conversion efficiency unless a speed-reducing gear box is interposed between the gas turbine and the generator.
It would therefore be desirable to provide a system by which a fossil fuel such as low-quality natural gas could be economically utilized in order to meet the growing demand for electricity. It would further be desirable to maximize the efficiency by which such fuels are converted into electrical energy within such a system by allowing the combustion turbine units to modulate their rotational speed freely in response to load requirements.